The enzymatic processes that produce different glyceryl esters have a selectivity which has not yet been fully elaborated. The specificities currently recognized for these enzymes do not account for all the known patterns of fatty acid composition observed in glycerolipids. In addition, the composition of the acyl chains in membrane lipids, which is controlled by the synthetic enzymes, appears to have marked modulatory influences on the functionality of membrane-associated phenomena. Experiments will focus on the relationships between acyltransferase specificities and the observed shifts in cellular lipid composition with selected functions of membranes that may be related to "fluidity" or fatty acid composition. Studies will be designed to indicate whether an acid that allows rapid and extensive cellular growth does so by its effectiveness as a biosynthetic substrate or by the effectiveness of its resultant products. These studies are designed to provide a fundamental insight into the metabolic process by which a saturated or unsaturated acid may be perceived by living cells. The successful extension of selective quantitative biosynthetic studies into cellular growth and function measurements allows a new appraisal of the role of lipids in modifying the fluidity of cellular membranes. Furthermore, the major differences in handling of different positional and geometrical isomers of octadecenoic acids raise new questions on the effects that the acids of partially hydrogenated vegetable oils may have upon cellular function. The above features, considered as a whole, may well be incorporated into an expanded interpretation of the role of unsaturated lipids in the etiology of heart disease.